Apbil, 1902.] 



KNOWLEDGE, 



79 



planet iu the latitude of Greece, it would seem a 

 most remarkable thing if the pkuet had not been seen 

 and notfd as tuch." Tliis proceeds partly from the 

 ulimates of Chaldea, Ej^ypt and Greece beinj^ far better 

 than that which we possess here in Euslaud. But in 

 .uldition to this the greater nearness to the equator of 

 these countries involving that the daily path of the sun 

 and planets was more nearly perpendicular to the horizon, 

 and that twdight was much shorter, gave thetn an 

 immense advantage over us for observing this planet. 



The interval between an east elongation of Mercury aud 

 tiie following west elongation is a little over six weeks, 

 Mercurv passing between the sun aud the eartli in the 

 meantime. The interval from west elongation to east 

 elongation. Mercury passing behind the sun, is a little 

 over ten weeks, the mean periods being 43i days and 72^ 

 davs respectively. The mean interval therefore from one 

 elongation to the same elongation again is llt5 days. 

 Since therefore Mercury goes through the complete cycle 

 of his positions with regard to the sun in a little under 

 four months, there must be at least one favourable con- 

 figuration in every year. In general tliere will bo two or 

 three. The attentive watcher therefore should have no 

 difficulty in catching sight of the elusive little planet at 

 at least one elongation in each year. 



The motions of Venus are similar in character to those 

 of Mercury, but are performed more slowly, and the arc 

 through which she swings is a wider one. The mean 

 l^eriod from east elongation as an evening star to west 

 elongation as a morning star is 1-13 days, whilst it is 

 441 days from west elongation round to east ; the entire 

 synodic jieriod therefore being 584 days. There is of 

 course no diflicnlty at all in recognising Venus when she is 

 near her elongations for she is then the most brilliant star 

 in the sky, but some interest attaches to her changes of 

 brightness. These changes depend upon two circum- 

 stances : the one, the distance which she is from the earth, 

 and consequently tlie aj>jiarent size of her disk ; the other, 

 upon the amount of that disk which is lighted up by the 

 sun, in other words upon her phase angle. The disk is 

 greatest when Venus is between the earth and the sun, but 

 at such a time the face she turns to us is necessarily in 

 darkness. On the other hand, when she is in superior 

 conjunction, that is to say on the further side of the sun, 

 her disk is entu'ely Uluminated, but it is then at its 

 smallest size In both cases she is invisible since she is 

 in full sunlight. 



At elongation her disk is much larger than at superior 

 conjunction, so much larger indeed that though it then 

 ap]»ears like a " half-moon," it gives us three times the 

 light which it would do could we observe it when it was 

 full. But as she passes from east elongation towards 

 inferior conjunction, the increase in apparent size com- 

 pensates, and more than compensates for the decrease in 

 phase, until half-way between the two positions her light 

 is four times that at superior conjunction. Prom this 

 point the effect of phase is greater than that of increase of 

 size. The time of greatest brilliancy therefore is some 36 

 days before inferior conjunction, and again 36 days after. 

 Then a period of 512 days will ensue before Venus returns 

 a second time to her greatest brilliancy as an evening star. 

 And as five times 584 days— the synodic period of Venus 

 — is almost exactly eight terrestrial years, it follows that 

 at mtervals of eight years the times of greatest brilliancy 

 recur on almost the same dates. 



At her greatest brightness, Venus is so brilliant that 

 there is no real difficulty iu seeing her with the naked eye 

 in full sunshine, or indeed at high noon. The hindrance 

 is pot the want of brightness of the planet, but the 

 difficulty in picking up so minute a point of light as she 



appears to be, unless some means is provided for guiding 

 the eye to the precise spot. When so found the first 

 impression is — " How could I possibly have overlooked so 

 bright an object ? " The next impression, if the eye be 

 turned from the planet but for a moment is — " What a 

 hopeless task it is to tr_y and find it again ! " 



The statement has sometimes been made that the phase 

 of Venus may be seen under exceptional conditions with 

 the naked eye. Frankly I think this observation lies out- 

 side the limit of possibility ; for Venus at her greatest 

 brilliance is only about 40 seconds of ai'c in diameter. 

 Now 40 seconds is practically the limit for distinct defined 

 vision, and it is very improbable that the precise shape of 

 Venus could be detected under such circumstances. It 

 would, however, be interesting to know if any one could 

 detect that the planet was not round. It is just possible 

 that an impression of elongation might be given, though 

 this would show exceptionally keen sight. 



The second group of the planets includes Saturn and 

 Jupiter, which both move in orbits far outside that of the 

 earth. From the point of view of the naked-eye astro- 

 nomer, neither is of very great interest. Their movements 

 do not greatly differ from those of the stars, amongst 

 which they move but slowly. Like the stars they will 

 have their heliacal risings, being seen as morning stars in 

 the east just before sunrise. Rising earlier and earlier 

 the time comes when they are visible for the entire night. 

 But before they are in opposition to the sun, that is to 

 say, are on the meridian at midnight, there is a striking 

 change in their apparent motion amongst the stars. For 

 the greater part of the year, Saturn is moving eastwards 

 amongst the stars at an average rate of a degree in about 

 eight days. Jupiter traverses the same distance in about 

 half that time ; but gradually as the time of opposition 

 draws on, the speed of both planets diminishes, until 

 Saturn comes to a stop about 70 days before opposition, 

 and Jupiter about 60. Then for 143 days Saturn moves 

 westward, and .fupiter for 122, both planets becoming 

 stationary again at the end of the period, and then 

 resuming once more their eastward march. This period 

 of westerly movement or retrogression marks tiie time 

 when the planet is nearest to the earth aud therefore 

 brii^'htesT, and it is at the middle of this period that the 

 planet is in opposition, that is to say, is on the meridian 

 at midnight. 



Just as it has been asserted that the crescent of Venus has 

 been seen with the naked eye. so it is also asserted, but on 

 somewhat better authority, that the satellites of Jupiter 

 have been seen at their elongations from their primary, 

 aud though of course it is utterly beyond the unaided 

 sight to perceive the ring of Saturn, the claim has Ijeen 

 made that Saturn has been observed as an elongated, not 

 as a circular point of light. By a curious coincidence it 

 happens that the diameter of Jupiter at opposition, the 

 major axis of Saturn's ring, and the diameter of Venus 

 at greatest brilliaucy, are all very nearly equal, and all 

 are very near the limit of defined vision. It follows there- 

 fore that the Saturn observation is as difficult as that of 

 Venus, but the satellites of Jupiter are not quite so hope- 

 less. The first and second are always too close to their 

 primary to be seen apart from it, and they are probably too 

 faint as well. But the third would be very readily visible 

 if it were a solitary star, and at its greatest elongation 

 from the planet it is distant from it 5j minutes of arc, 

 one-sixth of the apparent diameter of the moon. Many 

 people can separate e, and «, Lyrse which are con- 

 siderably nearer to each other. The fourth satellite 

 attains a distance at greatest elongation of nearly ten 

 minutes of arc, a distance amply sufficient to separate it 

 from Jupiter, but is by no means so bright an object as 



